Which Perl regular expression correctly replaces 'PG3' with 'PG3M6'?

The choice that correctly replaces 'PG3' with 'PG3M6' is represented by the syntax 's/PG3/PG3M6/'. This is because the 's' at the beginning indicates that this is a substitution operation in Perl's regular expression syntax. The full format of a substitution operation is 's/pattern/replacement/', which clearly identifies the pattern to match (in this case, 'PG3') and what to replace it with (in this case, 'PG3M6').

The other options employ either incorrect syntax for replacement or do not perform a substitution operation at all. The first option, while using 'r', does not conform to Perl's substitution rules. The third option represents a pattern matching operation rather than a replacement, and the fourth option would actually replace 'PG3M6' with 'PG3', which is the opposite of what is needed in this scenario. Thus, the substitution command with the 's' is the correct choice for performing the desired action of replacing 'PG3' with 'PG3M6'.